Update PrimitiveFieldType$STRING.java for UTF-8

Add support of UTF-8!
For ~i18n (eg. TTS).

### Docs
1. ["A string must always contain UTF-8 encoded or 7-bit ASCII text."](https://developers.google.com/protocol-buffers/docs/proto#scalar)
2. ["unicode strings are currently not supported as a ROS data type. utf-8 should be used to be compatible with ROS string serialization. "](http://wiki.ros.org/msg)

### Quote
It works. 
- testing with python node.
- for bug test, send string msg with ["éè"](http://www.utf8-chartable.de/unicode-utf8-table.pl?start=128&number=128&utf8=0x&unicodeinhtml=hex), on receive no Multibyte-char managed.
- check for best implement, just a fix!